The study of genes that influence cancer susceptibility is a rapidly evolving field. The power of mouse genetics coupled with the ability to scan entire genomes of individual animals has led to the discovery that several chromosomal regions harbor genes conferring susceptibility or resistance to different types of cancers. This proposal is focused on identifying and characterizing gcnes that influence the development of cancers in the gastrointestinal tract. Our goals are to use newly established congenic mouse lines to identify additional loci influencing cancer susceptibility. The system we have chosen involves the tumor suppressor gene Adenomatous Polyposis Coli (APC). Mutations in APC cause inherited and sporadic colorectal cancers. Apc Mi"mice have a mutation in the homologue of the APC gene and develop multiple adenomas throughout their small and large intestines. QTL studies identified a locus, Modifier of Min (Morn1), which maps to the distal region of chromosome 4 that dramatically modifies Apc_"-induced tumor number. We previously reported that the secretory type II Phospholipase A2 (Pla2g2a) gene is a strong candidate for Morn1. Inbred strains of mice display 100% concordance between Pla2g2a allele type and tumor susceptibility. Expression and sequence analysis revealed that Moml susceptible strains are null for Pla2g2a activity. We have established mice eongenic for the C57BL/6J Pla2g2a-/- region on the CAST/Ei resistant inbred strain background. Using this newly developed congenic strain in crosses wt.m., -A-pC Mint/_l_ mice, we will analyze offspring for several parameters measuring tumor phenotype and perform QTL analyses on the genome of offspring to identify additional loci that can influence polyp multiplicity. The identified regions of the genome will be further subjected to molecular dissection to determine their influence on cancer susceptibility; these studies will lead to the identification and characterization of gene(s) responsible for altering susceptibility. Ultimately, examination of newly identified modifier loci in human tumors should allow an understanding of the relationship between the effects of modifier genes on human tumor initiation, growth and progression. Further investigations will ultimately lead to insights regarding the value of these modifier genes in cancer diagnostics, prevention and treatment.